The SIMBOL-X hard X-ray optics

° SAIt 2008 c

The SIMBOL-X hard X-ray optics

G. Pareschi (on behalf of the SIMBOL-X collaboration)

INAF-Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy

Abstract. The SIMBOL-X formation-flight X-ray mission will be operated by ASI and CNES in 2013. Thanks to the formationflight architecture, it will be possible to operate a long (20 m) focal length grazing incidence optics module made of 100 confocal multilayer-coated Wolter I shells. This system will allow us to focus for the first time X-rays over a very broad energy band, from 0.5 keV up to 80 keV and beyond, with more than two orders of magnitude improvement in angular resolution and sensitivity compared to non focusing detectors used so far. In this paper, the SIMBOL-X optics design, technology and implementation challenges will be discussed.

Key words. Hard X–ray telescopes, X–ray astronomical optics, X–ray multilayer mirrors

1. Introduction

SIMBOL-X (Ferrando et al. 2004;

Pareschi and Ferrando 2006) is a hard X-ray mission based on a formation flight architecture, operating in the 0.5 – 80 keV energy range, for which a comprehensive Phase A study is being jointly carried out (at the time being almost concluded) by CNES and ASI, with a large participation of Italian (INAF), French (CEA, CNRS, APC) and and German (MPE, IAAT) scientists. SIMBOL-X makes uses of a long (20 m) focal length multilayer-coated X-ray mirrors, to focus for the first time X-rays with energies above 10 keV. The resulting improvement is of some three and two orders of magnitude in sensitivity and angular resolution respectively compared to the no focusing techniques used so far (basically collimated detectors or large open angle coded mask systems). The

mission will be operated on the second

half of 2013, by means of a Soyuz rocket

launched from Kouru. The SIMBOL-X

revolutionary instrumental capabilities will

allow us to elucidate outstanding questions

in high energy astrophysics, related in

particular to the physics and energetic

of the accretion processes ongoing in the

Universe. It will perform a census of black

holes on all scales, achieved through deep,

wide field surveys of extragalactic fields

and of the Galactic centre, and the to

the acceleration of electrons and hadrons

particles to the highest energies. The

SIMBOL-X optics is based on classical

Wolter I mirrors, to focus X-rays onto a

focal plane detector system. The gain in

terms of maximum observable energy is

achieved by the combination of a long

focal length (20m, i.e.'3 times larger than

XMM–Newton) and the use of broad-band

multilayer Bragg reflecting coatings (su-

permirrors). Since the large focal distance cannot fit in a single spacecraft, the mirror and detectors will be flown on two sepa- rate spacecrafts, using a formation flying configuration. In the following sections the main scientific and technical requirements related to the SIMBOL-X optics realiza- tion, together with the technological issues to be tackled, will be reviewed.

2. The mirror technology and issues The SIMBOL-X optics module will be based on pseudo-cylindrical monolithic Ni electroformed mirror shells with Wolter I profile. The adopted technology was al- ready successfully used for making the gold coated soft X-ray mirrors of the Beppo- SAX, XMM-Newton and Jet-X/Swift mis- sions (Fig. 1). This approach, developed in the past two decades in Italy by the INAF–

IASF (Milano) and INAF–Osservatorio Astronomico di Brera in collaboration with the Media Lario Technology com- pany (www.medialario.com), is well consol- idated. The industrial readiness for produc- tion is very high, being a number of the equipments (e.g. the lapping and metrology machines for mandrels, the electroforming baths, and the vacuum chambers for the gold coating deposition) already available from past projects (Vernani et al. 2008).

The diameter of the outermost mirror shell will be 65 cm, which is close to the max- imum size allowed with the present man- drel polishing technology (70 cm). Just a few modifications of the process will be im- plemented:

• the SIMBOL-X hard X-rays mirrors will rely on a very large focal length (20 m), allowed by the formation flight architecture. The integration equip- ments based on optical/UV at present available at Media Lario Technologies can fit a focal length just ≤ 8 m and, therefore, they need to be refurbished for SIMBOL-X.

• the SIMBOL-X mirror shells will make use of multilayer reflecting coatings,

Fig. 1. Mirror shells and mandrels developed for making the X-ray optics module aboard Swift.

instead of the usual Au layer (as e.g.

for XMM) or the Pt layer proposed in the preliminary SIMBOL-X study. The use of broad-band multilayer reflecting films not only will make possible to rely on a much larger FOV, but also extends the operative range up to 80 keV and beyond.

• Due to the need of maintaining the weight as low as possible, the Ni walls will be much thinner than the XMM mirror shells (' 2 less). At this re- gard, a new integration procedure (al- ready tested with success) will be imple- mented with the scope of maintaining good imaging performances, compliant with the top level requirements, even if the Ni shells will be much thinner and floppy than for past missions.

Regarding the multilayer coating, it will

be applied by adding an additional step to

Fig. 2. Scheme of the process, based on linear DC magnetron sources, to apply the multilayer coatings onto the surface of mirror shells.

the usual Ni replication process (see Fig.

2). Once that the gold-coated Ni mirror shell has been replicated from the man- drel, the multilayer film will be grown onto the internal surface of the shell by using a two-targets linear DC magnetron sputter- ing system, following the process developed ad hoc at the SAO–CfA for monolithic pseudo–cylindrical shells (Romaine et al.

2006). After a consolidation of the process in view of the SIMBOL–X optics imple- mentation, a multilayer coating facility for production will be installed at Media Lario.

The new assembly approach for thin floppy mirror shells is based on the use of two stiffening rings, to be applied at the top and bottom of each mirror element to restore an acceptable roundness profile and to handle the shells during the integration in the spiders (Basso et al. 2008). The two rings will be removed once that the shell is fixed by glue to the two spiders. In order to maintain a sufficient rigidity, their de- sign foresees a number of arms (24) larger than usual. Moreover, also the use of new materials for the mirror walls electroform- ing are being studied for SIMBOL-X, in order to reduce the deformations during the shell separation from mandrel and han- dling. At this scope particularly promising is the implementation of the Ni-Co alloy (already investigated by NASA-MSFC for the development of the Constellation–X op- tics) instead of pure Ni used so far, since it

presents more performing mechanical pa- rameters; in particular, the micro-yield is ' 5 times better, and the elasticity modu- lus ' 10 % higher.

It should be noted that another criti- cal point of the SIMBOL-X mirror produc- tion concerns the superpolishing process for mandrels. The surface profiles should have intrinsic errors similar to XMM (6 arcsec HEW on average) but with a very low roughness level, in order to minimize the scattering effects at high energies (a typical slope of the PSD of ∼ 2 in the range between 200 and 0.1 µm is needed (Spiga et al. 2008)). It has been already proven at INAF-OAB that these parameter can be achieved by using classical grinding and polishing techniques, but with a quite slow working time. In order to speed up the production time, the use of new figuring and lapping technologies are under investi- gation at Media Lario, based on the use of the single point diamond turning machin- ing and the magneto-rheological finishing.

3. The scientific requirements versus optics design

During the Phase A study the scientists of the Joint Scientific Mission Group, co- ordinated by ASI and CNES, worked out a document of top level requirements for SIMBOL-X, to be fulfilled for a complete compliance with the core scientific objec- tives of the mission. In this context, a num- ber of parameters directly concern the ca- pabilities of the optics module, as reported hereafter.

• On axis Effective Area

≥ 100 cm

at 0.5 keV, ≥ 1000 cm

at 2 keV, ≥ 600 cm

at 8 keV, ≥ 300 cm

at 30 keV, ≥ 100 cm

at 70 keV, ≥ 50 cm

at 80 keV (goal)

• Angular Resolution

≤ 20 arcsec (HPD, requirement), E <

30 keV ≤ 15 arcsec (HPD, goal), E <

30 keV ≤ 40(HPD) @ E = 60 keV (goal)

• Field of View

≥ 12 arcmin (diameter) @ 30 keV

Table 1. Angular resolution at 30 keV for past hard X-ray missions and SIMBOL–X

Experiment Year Imaging Technique Angular Resolution Parameter SAX-PDS 1996 Rocking Collimator ≥ 3600 arcsec collimator pitch INTEGRAL-IBIS 2002 Coded Mask ≥ 720 arcsec mask element pitch HEFT (balloon) 2005 Multilayer Optics ≥ 90 (30) arcsec HEW (FWHM)

SIMBOL–X 2013 Multilayer Optics 15 (' 5) arcsec HEW (FWHM)

Compared to past hard X–ray missions (Tab. 1), SIMBOL-X will have unprece- dented imaging capabilities. It should be noted that the very demanding require- ment in terms of angular resolution is mainly dictated by the exigency of avoiding problems of confusion among weak sources in deep surveys and to solve the puzzling problem of the hard X-ray emission from the Galactig Center region.

4. The optics design

The optics configuration of the SIMBOL- X optics has been addressed in the context of the on-going study Phase A study. After performing a trade–off aiming at a design compliant with the specifications but com- patible with the mass constraint of the mis- sion (the limit for the whole optics assem- bly, including structures and subsystems, was ≤ 480 kg) we arrived at the set of pa- rameters reported in Tab. 2.

The mirror shells, after the integration, are kept fixed to the mechanical case by the use of two spiders, a solution already adopted for BeppoSAX and JET–X/Swift (see Fig.

3). The weight of the sole mirror shells is 287 kg; the whole optics module (includ- ing also case, spiders, adapter, star trackers and other minor subsystems) is 413, plus a 10 % contingency. The theoretical on-axis effective area of the mirror shell is given in Fig. 4, while the expected field of view as a function of the photon energy is shown in Fig. 5. The overall telescope effective area is not only affected by the spider vi- gnetting, but also other factors contribute

Fig. 3. Drawing of the SIMBOLX optics module, including subsystems (pre-collimators and particle diverters); just a few representa- tive mirror shells are reported.

to its reduction. The most important ones

are the quantum efficiency of the detectors,

the presence of filters to protect the fo-

cal plane from optical stray–light, the area

lost to the detector segmentation and, fi-

nally, the presence of two Al coated thermal

blankets at the optics top and bottom, im-

plemented to make easier the thermal con-

trol of the system; at this regard it should

be noted that the temperature of the op-

tics should be berween 17 and 22

C, with

a gradient ≤ 1

C. The resulting effective

area, after taking into account all these ef-

fects, is depicted in Fig. 6.

Table 2. SIMBOL–X Optics Parameters

Number of modules 1

Geometrical profile Wolter I

Number of nested mirror shells 100

Reflecting coating Pt/C multilayer (200 bi-layers)

Focal Length 20000 mm

Total Shell Height 600 mm

Plate Scale 10.3 arcsec/mm

Material for the mirror walls Electroformed Ni (or Ni-Co)

Min-Max Top Diameter 260 to 650 mm

Min-Max angles of incidence 0.18 – 0.23 deg Min-Max wall thickness 0.2 to 0.55 mm Angular resolution HEW (goal) 20 (15) arcsec

Mirror shell weight 287 kg

Total optics module weight 413 kg

Additional mass contingency 40 kg

Fig. 4. SIMBOL-X expected onaxis effective area; the only vignetting due to the spiders arms is taken into.

5. Preliminary error budget tree A preliminary budget of the errors deter- mining the worsening of the HEW is re- ported in Tab. 3 for both requirement and goal levels. This evaluation has been done on the basis of the experience from previous projects (in particular XMM) and the pre- liminary results of the on–going assessment study. Apart from the errors concerning the mirror shells production and integra- tion, particular attention should be given to the problem of the focal length uncer- tainty (see Fig. 7), since different sources can contribute to this effect. Indeed a first

Fig. 5. Field of View (diameter at 50 % vi- gnetting of the on-axis effective area) as a func- tion of the photon energy. The cut determined by the size of the focal plane detectors is also reported.

issue comes from the use of two detectors

(the LED and the HED) in series with the

ideal focal position fall in between; for this

reason the distance between the two detec-

tion planes cannot be larger than a couple

of cm. Another problem is represented by

the ditering along the optical axis of he de-

tector spacecraft with respect to the mirror

spacecraft; the possible oscillations should

Fig. 6. SIMBOL-X onaxis effective area eval- uated taking into account all the effects con- tributing to the its decrease. The levels im- posed by the mission requirements are also re- ported.

Fig. 7. HEW increase due to the focal length error, assuming an intrinsic optics HEW of 15 arcsec (the two profiles represent different ap- proaches for the calculation).

be also kept at a level of a few cm at most.

6. Conclusions

The SIMBOL–X focusing optics will al- low us for the investigation of the hard Xray (≥ 10 keV) sky with unprecedented flux sensitivity and imaging capability. The formation–flight architecture makes possi- ble the implementation of long focal lenght

Table 3. Preliminary error budget tree for the optics angular resolution; since the er- rors are independent to each–other, the to- tal budged comes from their quadratic sum.

Error Source Requirement Goal (arcsec) (arcsec)

Mandrel profile 7 6

Mirror shell repl. 8.5 6

Single shell integr. 6.5 6

Xray scattering 14.8 10

Focal length err. 3 2

N shells integr. 2 2

Thermal deform. 2 1

Contingency 1.5 2.9

SQUARE SUM 20 15

(20 m) multilayer–coated mirrors in Wolter–I configuration, with an aperture similar to that of XMM but with an effec- tive area profile extended from 0.5 keV to beyond 80 keV. The technology adopted for the fabrication of the SIMBOL–X optics will be the well consolidated Ni electroforming replication, with some changes. A technological assessment study is currently on going, aiming at getting the technology ready for starting the activities of the flight optics module production on 2009.

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